Not legally without any sales or agreement papers. However, you can ask her to pay for all vet and boarding fees you may charge for his care before you release the horse to her. Chances are she'll forget the whole thing. Or you can just move with him, that will be very risky especially if she knows where you might be going. She can claim theft and have you arrested. Call the local Sheriff and ask questions, put everything you say as a hypothetical question. Don't put yourself in a bad situation. Go to her house with a witness at your side and make your intentions known. Are there gentlemans agreement laws in SC. Some states there are. Hope you luck, most of all I wish the horse luck.